Big Agnes footprints are made in sympathy with their tents - that means an ultralight tent gets a very lightweight footprint. An exact fit to the floor of the tent there's no surplus material or weight.

Big Agnes says:

To extend the life of your tent floor, we recommend using a Big Agnes footprint or ground cloth. The Big Agnes footprint also enables you to pitch a lightweight, Fast Fly shelter while leaving the tent body behind. The Fast Fly configuration requires the footprint, tent fly, poles, and stakes.

Most manufacturers recommend the use of a footprint to extend the life of the groundsheet of your tent, but if you've just bought an ultralight tent do you really want to carry the extra weight associated with it? And if you always need a footprint then why don't the manufacturers just beef up the floor fabric?

Our recommendation is to use a footprint if you expect to be camping on rough stony ground - Corsica's GR20 is a good example of this. Some footprints have other uses, such as being able to pitch just the flysheet and footprint together as an ultralight shelter - so in this case the footprint provides you with increased flexibility in your shelters. The footprint also does a great job of keeping the floor of the tent clean when your pitch is wet or muddy.

Finally, on Big Agnes in particular, they are good at minimizing the weight of the footprint so if you buy a tent from their ultralight range then you get an ultralight footprint - not all brands seem to understand the importance of this.